1. Field of the Invention
The present invention relates to a small-sized antenna capable of being incorporated in an apparatus such as a movable body communication apparatus, particularly to a small-sized antenna capable of receiving electromagnetic waves in a UHF television band with comparatively long wavelengths.
To be more specific, the present invention relates to an antenna configured in a small size while being improved in standing wave ratio in a used band by shifting a resonance point frequency into a lower band through a wavelength-shortening effect, particularly to an antenna configured in a small size by utilizing a wavelength-shortening effect obtained from a magnetic body having both a dielectric property and a magnetic property.
2. Description of the Related Art
In relation to portable radio apparatuses such as cellular phones, it is requested to further reduce the sets in size and weight while contriving enhanced functions. Accordingly, there has been an increasing demand for a reduction in the size of an antenna which is mounted on or in each of these apparatuses to perform transmission and reception (refer to, for example, “Antennas for Portable Apparatuses: Challenges for ‘Wide-band and yet Small’” (Nikkei Electronics, Nov. 22, 2004, pp. 69 to 80) (Non-Patent Document 1)).
An antenna, basically, is composed of a radiating element, a feeder line for feeding the radiating element with electricity, and a ground for grounding the radiating element. Here, a close relationship exists between the size of the antenna for transmission and reception, or the element length of the antenna, and the operating frequency. For example, in a monopole type antenna with a radiating element disposed on a ground, the element length is often set to be about quarter wavelength relevant to the operating frequency, from the viewpoint of efficiency or the like, even where a smaller size is contrived. Therefore, it is a common practice to contrive a smaller antenna size by shortening the wavelength of the electromagnetic field between the radiating element and the ground conductor.
For example, there has been known a dielectric antenna in which the element length is shortened by disposing the radiating element in proximity to a dielectric, paying attention to the wavelength-shortening effect possessed by the dielectric. This depends on the fact that the velocity of electromagnetic waves is lower in substances than in vacuum (refer to, for example, Hidetoshi Takahashi, Electromagnetics, p. 329, Butsurigaku Sensho 3, Shokabo, in 1970 (Non-Patent Document 2)). Let the frequency of an electromagnetic wave be f, let the velocity of the electromagnetic wave in vacuum be c, then the wavelength λ0 is represented by the following equation (1):
                              λ          0                =                  c          f                                    (        1        )            
On the other hand, the propagation velocity c of electromagnetic waves in vacuum and the velocity v of the waves in a substance are represented respectively by the following equations (2) and (3), where ∈0 and μ0 are the dielectric constant and the permeability in vacuum, and ∈r and μr are the relative dielectric constant and the relative permeability in the substance.
                    c        =                  1                                                    ɛ                0                            ⁢                              μ                0                                                                        (        2        )                                v        =                  1                                                    ɛ                0                            ⁢                              ɛ                r                            ⁢                              μ                0                            ⁢                              μ                r                                                                        (        3        )            
Since the wavelength λ of an electromagnetic wave with a frequency f in the substance is obtained as λ=v/f, its ratio to the wavelength λ0 in vacuum is given by the following equation (4).λ/λ0=(ν/f)×(f/c)=1/√{square root over (∈)}r  (4)
For example, the wavelength λ of the electromagnetic wave in a dielectric with a relative dielectric constant ∈r is represented by the following equation (5), which shows that the wavelength λ is made shorter than the wavelength λ0 in vacuum by a factor of 1/∈r due to the wavelength-shortening effect of the dielectric.
                    λ        =                              λ            0                                              ɛ              r                                                          (        5        )            
The dielectric chip antenna and the dielectric patch antenna, in which a radiating element is provided in the inside of a dielectric or at a surface of a dielectric, have recently been put to practical use in a variety of fields, as a small-sized transmission/reception antenna mainly for the GHz band.
Besides, the wavelength λ of the electromagnetic wave propagated through a magnetic body with a relative dielectric constant ∈r and a relative permeability μr is represented by the following equation (6). In other words, a magnetic body having both a dielectric property and a magnetic property can shorten the wavelength further than in the case of using a dielectric, by a factor of 1/μr.
                    λ        =                              λ            0                                                              ɛ                r                            ·                              μ                r                                                                        (        6        )            
According to the above equation (6), in principle, a material having a relative dielectric constant of 5 and a relative permeability of 5, for example, may show a wavelength-shortening effect equivalent to that a dielectric having a relative dielectric constant of 25. However, as of the time of the present application, examples of application of magnetic bodies to antennas have been limited; even ferrites showing low losses at high frequencies have been used as bar antennas for AM broadcast receivers, and there have been known few examples of application in frequency regions at or above the MHz band. In the case where a magnetic body has a dielectric property as well, both a magnetic loss and a dielectric loss will be generated, leading to a lowering in radiation efficiency (refer to, for example, Japanese Patent Laid-open No. 2004-7510, paragraph No. 0006 (Patent Document 1)).
Recently, there have been a few examples of investigation made on magnetic materials from the viewpoint of contriving a reduced antenna size. It has been suggested, for example, that when a simulation with material characteristics as parameters is conducted and a magnetic body satisfying certain conditions is used, a reduction in the size of a patch antenna or helical antenna can be achieved (refer to, for example, Hiromu Sumi, “A Study on Antennas Utilizing Magnetic Materials”, a graduation thesis in the Department of Engineering of Yokohama National University in the class of 2002 (Non-Patent Document 3)).
Besides, it has been reported that when a flat plate inverse F antenna of 55 mm by 40 mm in size for 900 MHz band is used with the antenna substrate replaced by a magnetic body, the size thereof can be reduced to about 34 mm×30 mm, i.e., a reduction of up to about 50% in area ratio can be achieved (refer to, for example, Tomoteru Tanaka, Shogo Hayashida, Kazushi Imamura, Hisashi Morishita, and Yoshio Koyanagi, “An Investigation on Reduction in Size of Portable Terminal Antenna using Magnetic Material” (the Institute of Electronics, Information and Communication Engineers, Ronbunshi B, Vol. J87-B, No. 9. pp. 1327 to 1335, 2004) (Non-Patent Document 4)).
However, in the case of replacing the substrate of an antenna by a magnetic body, the shape of the antenna is limited to flat plate-like shapes. In addition, where reception of television broadcast in a UHF band at much lower frequencies, for example, about 500 to 800 MHz is considered, the occupying area is expected to be naturally larger than that in the above-mentioned report. Therefore, where mounting of antennas on portable apparatuses is considered, development of a technology for further reductions in the antenna size is desired.